re-add support for sourcing endpoint URLs from service-specific env config (#3568)

The service env config work I did was flawed in that it didn't respect
the precedence of programmatic config. This PR
adds a way of tracking the precedence up to the point of converting the
SdkConfig into a service config. Now, env config will only be resolved
when config was not set programmatically. I added tests to `aws-config`
for the origin tracking to ensure it works.

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choice._

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Co-authored-by: ysaito1001 <awsaito@amazon.com>